1. Field of the Invention
The present invention relates generally to high temperature alloys and, more particularly, to nickel-base alloys which are suitable for use in high temperature oxidizing and nitrogen bearing atmospheres.
2. Description of the Related Art
Performance requirements for thermal processing equipment and their components are dramatically increasing as industry strives for increasing productivity, cost savings, longer service lives and greater levels of reliability and performance. These requirements have motivated alloy manufacturers to upgrade the corrosion resistance, stability and strength of their alloys used in thermal processing applications while at the same time improving hot and cold workability in order to improve product yield and reduce cost to the consuming industry. These demands are particularly strong in a number of areas, including the powder metallurgy and silicon chip industries, the manufacture of thermocouple sheathing and protection tubes and in the resistive heating element manufacture. Wire mesh belting is an example of the type of application for which this alloy range is desired. In the power metallurgy (P/M) industry, metal powder is compacted in dies in the desired shape of a component and then sintered by exposing the compacted component in a controlled atmosphere at high temperature for a period of time. It is well-known that iron powders can be sintered to higher strength when sintered at increasingly higher temperatures. In addition, certain materials, notably stainless steels, requireextremely high temperatures (about 1200xc2x0 C.) to achieve useful corrosion and strength properties. These higher temperatures make the commonly used wire mesh belting alloy (Type 314 stainless steel) unacceptable for use due to lack of strength and high temperature nitridation resistance. A similar situation exists in the annealing of silicon chips at these temperatures, where spallation of the wire mesh belting must be as low as possible in order not to contaminate the silicon chips. Again, the spallation rate of commercial wire mesh belting alloys in this annealing atmosphere is deemed excessive and requires a marked improvement in corrosion resistance without loss of strength.
Commercial alloys commonly used as the sheathing alloy of mineral-insulated metal-sheathed (MIMS) thermocouples contain elements that ultimately at elevated temperatures degrade thermocouple (both K and N Type) performance by diffusing from the sheathing through the insulating mineral and reacting with the thermocouples to cause EMF drift. Certain alloys designed to resist this type of degradation while retaining adequate oxidation corrosion resistance have been found to be extremely difficult to manufacture in good yield.
Surprisingly, it has been discovered that the necessarily low spallation and metal loss rates, strength, stability and fabricability for the above industry requirements can be obtained by an alloy of the present invention having the following composition, in % by weight, about: 15.0-23.0% Cr, 0.5-2.0% Si, 0.0-4.0% Mo, 0.0-1.2% Nb, 0.0-3.0% Fe, 0.0-0.5% Ti, 0.0-0.5% Al, 0.0-0.3% Mn, 0.0-0.1% Zr, 0.0-0.06% Ce, 0.005-0.025% Mg, 0.0005-0.005% B, 0.005-0.3% C, 0.0-20.0% Co, balance Ni. Maximum strength, spallation and metal loss rates, and resistance to degradation of thermocouples can be obtained by restricting the alloy range further to a more preferred range consisting essentially of about: 21.0-23.0% Cr, 1.3-1.5% Si, 2.5-3.5% Mo, 0.0-0.2% Nb, 0.0-1.0% Fe, 0.0-0.1% Ti, 0.0-0.1% Al, 0.0-0.1% Mn, 0.0-0.1% Zr, 0.015-0.035% Ce, 0.005-0.025% Mg, 0.0005-0.005% B, 0.005-0.05% C, balance Ni. As used hereinafter, all % values, unless otherwise noted, are % by weight.
Normally, the above-described combination of elements would not be expected to perform all the aboveiscussed requirements within a single composition. However, it has been discovered that by using trace amounts of certain elements (Zr, Ce and Mg), the negative effects of certain other elements (Mo, Nb, Fe, Mn and Ti) can be ameliorated, and by restricting other elements to critically essential levels (Si, Al, B and C), their benefit can be utilized without degrading other properties. These balanced levels must be incorporated within a thermodynamically stable matrix which can best be found within the Nixe2x80x94Cr system when elevated temperature, strength and corrosion resistant properties must be maintained. Too often, striving for maximized strength or corrosion resistance results in alloys that cannot be commercially made economically or in large quantity on commonly used alloy manufacturing equipment. This impediment has been overcome by the alloy range of the present invention. The selection of each elemental alloying range can be rationalized in terms of the function each element is expected to perform within the compositional range of the invention. This rationale is explained in greater detail hereinafter.